1. Field of the Invention
This invention relates to an analog-digital broadcast shared receiving tuner, and particularly to an analog-digital broadcast shared receiving tuner capable of reducing the interference of a high-level analog broadcast of an adjacent channel upon reception of a low-level digital broadcast assigned to a vacant channel of an analog broadcast.
2. Description of the Related Art
In recent years, a digital broadcast came into service in a television-broadcasting field in addition to an analog broadcast which has been performed up to now.
Described specifically, a low-level digital broadcast is assigned to a vacant channel of an analog broadcast to transmit an analog-digital mixed broadcast.
Meanwhile, a double conversion type analog-digital broadcast shared receiving tuner having two frequency conversion stages so as to be able to receive a digital broadcast in a satisfactory state has been used as an analog-digital broadcast shared receiving tuner used for receiving such an analog-digital mixed broadcast.
FIG. 3 is a diagram for describing an example of a configuration of such an analog-digital mixed broadcast. FIG. 4 is a block diagram showing one example of the already-known double conversion type analog-digital broadcast shared receiving tuner.
As shown in FIG. 3, the analog-digital mixed broadcast is one represented in a signal form in which a digital broadcast channel D is assigned to a vacant channel between analog broadcast channels A.sub.1 and A.sub.2. The level of the digital broadcast is set so as to reach a low level of about 10 dB with respect to the level of the analog broadcast to eliminate interference with the analog broadcast.
Further, as shown in FIG. 4, the double conversion type analog-digital broadcast shared receiving tuner comprises an automatic gain control high-frequency amplifier 41, a first frequency conversion stage 42 comprised of a first frequency mixer 43 and a first local oscillator 44, a first intermediate frequency signal pass filter 45, a first intermediate frequency signal amplifier 46, a second frequency conversion stage 47 comprised of a second frequency mixer 48 and a second local oscillator 49, a second intermediate frequency signal pass filter 50, a second intermediate frequency signal amplifier 51 and a second intermediate frequency signal output terminal 52. A receiving antenna 53 is electrically connected to the input side of the automatic gain control high-frequency amplifier 41.
The analog-digital broadcast shared receiving tuner constructed as described above is roughly operated as follows:
When an analog-digital broadcast (50 MHz to 800 MHz) is received by the antenna 53, the automatic gain control high-frequency amplifier 41 effects automatic gain control amplification on the received high-frequency signal in response to a gain control voltage applied across a gain control terminal 41c and supplies the amplified signal to the first frequency conversion stage 42 subsequent to the automatic gain control high-frequency amplifier 41. In the first frequency conversion stage 42, the first frequency mixer 43 frequency-mixes the high-frequency signal into a first local oscillation signal supplied from the first local oscillator 44 and supplies the frequency-mixed signal to the first intermediate frequency signal pass filter 45 subsequent to the first frequency mixer 43. The first intermediate frequency signal pass filter 45 extracts only a first intermediate frequency signal (1 GHz as an example) from the frequency-mixed signal and supplies it to the first intermediate frequency signal amplifier 46 subsequent to the first intermediate frequency signal pass filter 45. The first intermediate frequency signal amplifier 46 amplifies the first intermediate frequency signal to a predetermined level and supplies it to the second frequency conversion stage 47 subsequent to the first intermediate frequency signal amplifier 46. In the second frequency conversion stage 47, the second frequency mixer 48 frequency-mixes the first intermediate frequency signal into a second local oscillation signal supplied from the second local oscillator 49 and supplies the frequency-mixed signal to the second intermediate frequency signal pass filter 50 subsequent to the second frequency mixer 48. The second intermediate frequency signal pass filter 50 extracts only a second intermediate frequency signal (44 MHz as an example) from the frequency-mixed signal and supplies it to the second intermediate frequency signal amplifier 51 subsequent to the second intermediate frequency signal pass filter 50. The second intermediate frequency signal amplifier 51 amplifies the second intermediate frequency signal to a predetermined level and supplies it to an available circuit (not shown) through the second intermediate frequency signal output terminal 52.
In this case, the frequency of the first local oscillation signal of the first frequency conversion stage 42 is selectively adjusted in the analog-digital broadcast shared receiving tuner so that a broadcast for one broadcast channel of the analog broadcast channels A.sub.1 and A.sub.2 or the digital broadcast channel D is received.
The already-known analog-digital broadcast shared receiving tuner has an advantage that the analog and digital broadcasts can be selectively received without individually providing an analog broadcast receiving portion and a digital broadcast receiving portion. However, a problem arises in that when it is desired to set the reception sensitivity of the analog broadcast by the already-known analog-digital broadcast shared receiving tuner substantially equal to the receiving sensitivity of this type (single conversion type) analog broadcast receiving tuner having one frequency conversion stage, interference components resultant from the analog broadcast adjacent to the digital broadcast channel D enter into the digital broadcast upon reception of the digital broadcast, so that the received digital broadcast is greatly degraded in quality.